Did my full IR in a duchess with a Garmin 430 fitted, but only one of my three instructors showed me anything about it (although I had read the manual) and the general advice I had was not to use it at all for the test.
It's funny, I also have found a real reluctance from instructors to use the GPS panel mounted units, whilst teaching, or recurrent training. (IR).
It is almost as if it is taboo to use any GPS derived aids. I also agree, a lot do not seem that proficient in it. One reason I have thought about is that most panel mounted units are on the right side, and all scan instruments are on the LHS. If concentrating on scan, then the distraction of coming away from the practiced scan, to look over at the GPS, can be awkward. Of course, a lot of the newer glass installs, have the GPS display in front of the LHS pilot.
Might not have anything to do with it though.